The present invention relates in general to direction finders and in particular to a new and useful acoustic correlation direction finder which utilizes a rotating base having a plurality of sound pickups which generate a plurality of signal including portions caused by sound from a target, which signals are correlated and otherwise processed to determine the direction of the target.
In acoustic correlation direction finding, acoustic signals coming from a target are received by a direction finding base, e.g. having two microphones or having four microphones arranged in the corners of a tetrahedron. In the correlation, acoustic signals which do not originate from a signal source but are distributed at random, such as background noise like rain, wind or the like, are averaged out, whereas the signals originating from a localizable signal source are summed by integration over time, so that there appear in the correlation function correlation peaks singularly correlated with these signal sources. The correlation peaks indicate the value of the time shifts at which the signals impinging on the microphones from the signal source are received. From this time shift the direction of the signal source relative to the microphone arrangement is determined. Additional details can be found in U.S. Pat. No. 4,236,159.
Acoustic correlation direction finders are used when acoustic signal sources are to be localized, as for instance in the military sector for the localization of armored vehicles (tanks).
Tanks are attacked, among other ways with so-called drop ammunition from the air. Such ammunition is ejected from an airplane or from an ammunition receptacle and then drops slowly, e.g. at first attached to a parachute and then braked by the air, to the ground, generally rotating about a vertical axis for stabilization. The drop ammunition has a search head, as well as control drives addressed by the search head signals, which drives will, after a target has been acquired, displace the drop ammunition transversely in the direction of the target. It would be desirable if the target search head could be equipped with an acoustic correlation direction finder, because, compared with electromagnetic direction finders as used otherwise, e.g. radar or "ladar" equipment, it is of less complicated construction and is cheaper as well.
In acoustic correlation direction finding with a rotating direction finding base, however, a problem arises. On the one hand, the correlation time must be very short, so that during that time the position of the direction finding base relative to the direction of sound impingement will change little. On the other hand, however, with shorter correlation time also, the signal processing yield and hence also the direction finding range decreases.